The global market for efficient power generation equipment has been expanding in recent years and is anticipated to continue to expand in the future. The gas turbine combined cycle power plant is a preferred choice for this type of equipment due to relatively low plant investment costs and continuously improving operating efficiency of the gas turbine-based combined cycle which minimizes electricity production costs.
It is well known that elevated firing temperature in the gas turbine is a key element in providing higher output per unit mass flow, enabling increased combined cycle efficiency and that for a given firing temperature, there is an optimal cycle pressure ratio which maximizes combined cycle efficiency. The optimal cycle pressure ratio trends higher with increasing firing temperature. Compressors for these turbines are thus subjected to demands for higher levels of pressure ratio consistent with other goals, such as minimal parts count, operational simplicity and low overall cost. This optimal level of cycle pressure ratio requires improved compression efficiency in the compressor, recognizing that the compressor must perform in an aerodynamically and aeromechanically stable manner under a wide range of mass flow rates associated with varying power output characteristics of the combined cycle operation.
The maximum pressure ratio that the compressor can deliver in continuous duty is commonly defined in terms of a constant margin in pressure ratio from the surge pressure ratio line. Presently, the surge line cannot be analytically determined with certainty nor can the aeromechanical response of the blading at pressure ratios near the surge line be determined. Analytical estimates of the location of the surge line must therefore be experimentally verified through compressor testing or compressor mapping tests. As will be appreciated, compressor surge is that low frequency oscillation of flow where the flow separates from the blades and reverses flow direction through the machine, i.e., it serves as a physical limit to compressor operation at a given speed.
Full-scale mapping of a compressor including the surge line has been previously accomplished over the entire range of operating conditions of the compressor. That mapping facility included, among other things, a compressor inlet throttling system; a reduced turbine first-stage nozzle area, and compressor discharge valves for bypassing compressor discharge flow about the turbine, as well as ancillary features. In that facility, the inlet throttling system employed a valve to regulate and reduce the flow of inlet fluid to the compressor inlet. The reduction in the first-stage turbine nozzle throat area was effected from production hardware by reforming the trailing edge at different angles. These and other modifications permitted attainment of high-pressure ratios above the nominal operating line by a slow steady-state approach to the surge line without over-firing the turbine. Details of that facility are described in an article titled "CTV--A New Method for Mapping a Full Scale Prototype of an Axial Compressor," authored by Vasco Mezzedimi, Pierluigi Nava and Dave Hamilla and appearing in an ASME article published in 1996, the disclosure of which is incorporated herein by reference. In efforts to map the surge line of a further compressor, it was found using that system, however, that the reduction in first-stage turbine area was inadequate to permit attainment of surge pressure ratios before limits on exhaust temperature were encountered. That is, before the surge pressure ratio could be obtained, the maximum operational exhaust temperature for the turbine was reached and this could not be exceeded without damage to the machine.